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Scientists develop phase-changing robot material

Lance Ulanoff

Two 3D-printed soft, flexible scaffolds: the one on the left is maintained in a rigid, bent position via a cooled, rigid wax coating, while the one on the right is uncoated and remains compliant (here, it collapses under a wrench). Photo: MIT

Scientists have developed a new material that could give future robots shape-shifting capabilities akin to the T-1000 robot in Terminator 2: Put that in your cigar and chew on it for a while, Arnold Schwarzenegger.

The 3D-printed, phase-changing material is actually a combination of polyurethane foam and wax. The developers — Massachusetts Institute of Technology Professor Anette Hosoi, her former graduate student Nadia Cheng, researchers at the Max Planck Institute for Dynamics and Self-Organization and Stony Brook University — figured out how to saturate the foam in wax and then heat and cool specific parts to transform them from rigid to soft and pliable and then back to rigid and hard again.

The applications are fairly obvious. A robot made of such material could soften up when it needs to move into a small space, but then regain its original shape and rigidity when it needs to, say move or lift an object.

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For their research, Hosoi and Cheng were trying to build a snake-like robot that could squeeze through a one-centimeter hole, emerge on the other side and still be able to move around. MIT researchers liken this capability to the way mice can squeeze though pencil-sized holes, but still come out looking and functioning like mice on the other side.

Soft robotics is nothing new, but the ability to — through electric current that generates heat — shift between two states, especially at such a low cost, is new. And the combination of foam and wax is unusual. Cheng did tell us that Harvard's Whitesides Research Group is working on wax-infused fabric that could also lead to self-healing robots.

When we asked Cheng, who now works on squishy robots for Empire Robotics, why the team used foam and wax, she said there really wasn’t a "eureka" moment.

"We wanted something that would produce significant volume change, but something that could still be useful, support payloads and enforce payloads on the environment," Cheng said. The solution could not be something that’s "completely squishy and floppy."

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Foam can change volume in three dimensions; it's widely available, and anyone can play with it. It has the added benefit of not conducting heat too widely. So, even as the team used copper wire coiled around the wax-infused foam to heat the robot, the heat would stay contained to one small area and dissipate as soon as the current was turned off.

The selection of wax came only after the rejection of the moreTerminator 2-like solder. It too can change form based on heat — from hard solid to liquid and back again — and even offers higher stiffness and rigidity than wax. Unfortunately, foam really can't absorb it in the same way.

With the materials settled, Hosoi and Cheng went to the craft shop for foam and Batik wax. However, they soon found that 3D printing the polyurethane foam gave them far greater control over the foam's struts and pores, which makes the entire structure more tunable.

That tuning happens through heating specific portions of the wax-infused foam. A larger robot built of such material might have just a forearm heated and softened so it could squeeze it into small area. The hand, perhaps made of the same material, would also soften for the trip, but stiffen (by cooling to room temperature) so it can grab something in the open space.

For now, though, the largest shape-shifting robot the team produced was only a few centimeters in size. Not to mention there are other hurdles to clear: The wax still takes a few seconds to heat and cool and, Cheng told us, they could not figure out how to contain the wax within the robot.

Guess Mr. Schwarzenegger is safe, for now.

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